An underground cavity leached out from salt may be several hundreds of meters tall starting from a depth which is also several hundreds of meters, with the diameter being typically several tens of meters. For example, an existing group of cavities extend between -900 m to -1500 m below ground level, and have varying diameters in the range of from 50 m to 80 m. Normally such a cavity is full of salt water or brine. Technically, it is possible to fill such a cavity with waste after it has been completely emptied by injecting gas (air, nitrogen, etc.) therein. However, this method poses numerous problems, and, in particular, it is very expensive because of the great depths at which work is performed.
The cavity can be emptied by pumping and injecting a gas under atmospheric pressure, but in that case the mechanical integrity of the cavity may be in danger because of the plastic properties of salt: salt is subject to creep. In order to increase the gas pressure sufficiently to oppose creep, it would be necessary to provide very high power compressors and to use very large quantities of gas. Such cavities frequently have volumes of several tens of thousands of cubic meters, and pressures of more than 100 bars are often required.
It would therefore be necessary to remove the brine by displacing it by means of the waste. However this solution is not very satisfactory since the extracted brine would be contaminated by contact with the waste, and this would prevent it from being dumped in the natural environment or subsequently used for commercial purposes, and would give rise to problems relating to protection of the environment.
Further, after the cavity has been filled, if the waste is in the form of a liquid or a semi-solid, or in the form of solid grains, it behaves like a fluid and transmits the geostatic pressure at the bottom of the cavity up to the top of the cavity. This pressure is high and is poorly withstood by the top of the cavity. This can give rise to fractures with the waste being put into communication with underground water above the cavity.
The object of the present invention is to provide a method of confining waste in an underground cavity leached out from salt, but avoiding the drawbacks of the prior art as described above.